Baseline characteristics of the study population (N = 41,102)
| Participant characteristic | |
|---|---|
| Age (years), mean (SD) | 55.0 (9.8) |
| BMI (kg/m2), mean (SD) | 25.4 (4.2) |
| Age at menarche (years), mean (SD) | 13.1 (1.5) |
| Missing | 1.9 (793) |
| Parity, % (N) | |
| 0 | 12.1 (4,979) |
| 1 | 14.1 (5,792) |
| 2 | 48.4 (19,901) |
| ≥3 | 25.2 (10,363) |
| Missing | 0.2 (67) |
| Age at first birth (years), mean (SD) | 27.1 (5.2) |
| Missing, % (N) | 5.6 (2008) |
| Menopausal status, % (N) | |
| Premenopausal | 40.2 (16,506) |
| Perimenopausal/unknown | 3.3 (1,349) |
| Postmenopausal | 56.5 (23,247) |
| Ever OC use, % (N) | 79.3 (32,602) |
| Missing | 0.5 (201) |
| HRT use, % (N) | |
| Never | 74.9 (30,791) |
| Former | 14.5 (5,945) |
| Current | 3.7 (1,521) |
| Missing | 6.9 (2,845) |
| Benign breast disease, % (N) | 21.7 (8,919) |
| Missing | 2.0 (811) |
| Family history breast cancer, % (N) | 12.7 (5,236) |
| Missing | 3.6 (1,471) |
| Total breast volume (cm3), median (IQR) | 760 (493–1,106) |
| Absolute dense volume (cm3), median (IQR) | 57 (42–77) |
| Percent dense volume (%), median (IQR) | 7.7 (5.3–11.7) |